1. Field of the Invention
This invention relates to the field of information networks, and more particularly relates to a protocol for configuring routes over a network.
2. Description of the Related Art
Today's networks carry vast amounts of information. High bandwidth applications supported by these networks include streaming video, streaming audio, and large aggregations of voice traffic. In the future, these demands are certain to increase. To meet such demands, an increasingly popular alternative is the use of lightwave communications carried over fiber optic cables. The use of lightwave communications provides several benefits, including high bandwidth, ease of installation, and capacity for future growth.
The synchronous optical network (SONET) protocol is among those protocols employing an optical infrastructure. SONET is a physical transmission vehicle capable of transmission speeds in the multi-gigabit range, and is defined by a set of electrical as well as optical standards. SONET's ability to use currently-installed fiber optic cabling, coupled with the fact that SONET significantly reduces complexity and equipment functionality requirements, gives local and interexchange carriers incentive to employ SONET. Also attractive is the immediate savings in operational cost that this reduction in complexity provides. SONET thus allows the realization of a new generation of high-bandwidth services in a more economical manner than previously existed.
SONET networks have traditionally been protected from failures by using topologies that dedicate something on the order of half the network's available bandwidth for protection, such as a ring topology. Two approaches in common use today are diverse protection and self-healing rings (SHR), both of which offer relatively fast restoration times with relatively simple control logic, but do not scale well for large data networks. This is mostly due to their inefficiency in capacity allocation. Their fast restoration time, however, makes most failures transparent to the end-user, which is important in applications such as telephony and other voice communications. The existing schemes rely on 1-plus-1 and 1-for-1 topologies that carry active traffic over two separate fibers (line switched) or signals (path switched), and use a protocol (Automatic Protection Switching or APS), or hardware (diverse protection) to detect, propagate, and restore failures.
A SONET network using an SHR topology provides very fast restoration of failed links by using redundant links between the nodes of each ring. Thus, each ring actually consists of two rings, a ring supporting information transfer in a “clockwise” direction and a ring supporting information transfer in a “counter-clockwise” direction. The terms “east” and “west” are also commonly used in this regard. Each direction employs its own set of fiber optic cables, with traffic between nodes assigned a certain direction (either clockwise or counter clockwise). If a cable in one of these sub-rings is damaged, the SONET ring “heals” itself by changing the direction of information flow from the direction taken by the information transferred over the failed link to the sub-ring having information flow in the opposite direction.
The detection of such faults and the restoration of information flow thus occurs very quickly, on the order of 10 ms (for detection) and 50 ms (for restoration) for most ring implementations. The short restoration time is critical in supporting applications, such as current telephone networks, that are sensitive to quality of service (QoS) because such short restoration times prevent old digital terminals and switches from generating red alarms and initiating Carrier Group Alarms (CGA). These alarms are undesirable because such alarms usually result in dropped calls, causing users down time aggravation. Restoration times that exceed 10 seconds can lead to timeouts at higher protocol layers, while those that exceed 1 minute can lead to disastrous results for the entire network. However, the price of such quickly restored information flow is the high bandwidth requirements of such systems. By maintaining completely redundant sub-rings, an SHR topology requires 100% excess bandwidth.
An alternative to the ring topology is the mesh topology. The mesh topology is similar to the point-to-point topology used in inter-networking. Each node in such a network is connected to one or more other nodes. Thus, each node is connected to the rest of the network by one or more links. In this manner, a path from a first node to a second node uses all or a portion of the capacity of the links between those two nodes.
Networks based on mesh-type restoration are inherently more capacity-efficient than ring-based designs, mainly because each network link can potentially provide protection for fiber cuts on several different links. By sharing the capacity between links, a SONET network using a mesh topology can provide redundancy for failure restoration at less than 100% of the bandwidth capacity originally required. Such networks are even more efficient when traffic transits several links. However, restoration times exhibited by such approaches range from several minutes to several months.